1. Fields of the Invention
The present invention relates to a method for manufacturing an absorber layer of thin film solar cells, especially to a method for manufacturing an absorber layer of thin film solar cells that controls flow rate of respective element and deposits the respective element to form the absorber layer.
2. Descriptions of Related Art
Compared with monocrystalline silicon solar cells or polycrystalline silicon solar cells, thin-film solar cells have been attracted much attention and considered the next generation of solar cells due to low cost and potential for use in large-area applications.
Main materials for thin-film solar cells include Copper Indium Gallium Diselenide (CuInGaSe2, CIGS), Copper Indium Diselenide (CuInSe2, CIS), Cadmium Telluride (CdTe), etc. Yet indium and gallium in CIGS are rare elements with high costs while selenium is toxic. Thus zinc and tin which are more abundant on earth and less-toxic sulfur are used to replace indium, gallium and selenium respectively. Thus copper zinc tin sulfide (Cu2ZnSnS4, CZTS) is used to produce an absorber layer of thin-film solar cells.
No matter CIGS or CZTS thin-film solar cells, most of them are prepared by coevaporation or sputtering under vacuum processes. There is room for improvement of energy conversion efficiency of the products.
Refer to FIG. 1, in the coevaporation method, four sets of solid evaporation sources 7 for copper (Cu), zinc (Zn), tin (Sn), sulfur (S), are heated to have changes in physical phase such as sublimation, evaporation, and deposition on a substrate 31 inside a reaction chamber 3 to form a thin film solar cell 5. Moreover, a heater 33 for heating and a monitor 6 for monitoring forming process of the thin film solar cell 5 are also arranged inside the reaction chamber 3.
However, the manufacturing process of the coevaporation method is a single stage process and all reactants reacts at the same time. The reactants are vapors formed by solids being heated and evaporated. Yet the boiling point and the vapor pressure of each reactant are different and this results in difficulty in control of respective quantity. And the vapors may be not mixed well and thus the uniformity and quality of the thin film are affected. The reproducibility of large-area production is reduced.
As to other methods such as sputtering, metal or metal chalcogenide precursors are produced in advance. Then the precursors are set into an environment containing S/Se for high-temperature sulfurization/selenization. The disadvantage of this method is in that this is a two-stage process that incurs additional production cost. This is a great obstacle that needs to be overcome otherwise the cost of the solar cell remains high and this has impact on promotion of the thin film solar cells. Moreover, metals may be evaporated during sulfurization/selenization and this affects the reaction ratio of the reactants. Furthermore, the different stacking order of the metal precursors leads to different timing of sulfurization. Thus there are some secondary phase substances produced including copper sulfide, zinc sulfide, and copper tin sulfide (Cu2SnS3). The energy conversion efficiency of the products is reduced.